Seasonality in precipitation stable isotopes and isotope hydroclimatological signals in surface- and subsurface waters: comparative study from a Hungarian and Slovenian catchments

Keywords: precipitation, river water, subsurface water, climate conditions, δ¹⁸O, δ²H, ¹⁴C groundwater ages

This study presents a comparative isotopic hydrological assessment of two continental catchments: the Koppány Stream in Hungary and the Ledava Stream in Slovenia. Stable oxygen (δ¹⁸O) and hydrogen (δ²H) isotopic signatures were monitored to investigate water source contributions, flow paths, and residence times under varying hydrological conditions.

The Koppány catchment, located in southwestern Hungary, spans approximately 660 km², with dominant agricultural land use and patches of natural forest. The Ledava catchment, extending across the Hungary-Slovenia border, covers 1,940 km² and shares similar climatic characteristics but features more lowland areas. Annual precipitation averages are 630 mm for Koppány and 798 mm for Ledava.

From 2022 to 2024, δ¹⁸O and δ²H measurements were conducted on precipitation, surface- and subsurface water (shallow and deep groundwater) in both catchments. Results show comparable isotopic values in precipitation across the two sites. In both catchments, river and groundwater exhibit smaller isotopic variability than precipitation. Subsurface water is more depleted than river water and precipitation, especially in deeper aquifers.

In the Ledava catchment, river water displays less negative isotopic values than mean precipitation, with an upstream (Polana) station showing slightly enriched signatures compared to a downstream (Čentiba) station. Shallow groundwater aligns well with river water isotopic composition, while deeper groundwater is more depleted but not as strongly as in the Koppány catchment. In the Koppány catchment, the seasonality of the stable isotopes in the streamwater was compared to the amplitude of precipitation. The study revealed the effect of the reservoirs on the evaporation and the presence of waters of different origins in the stream.

The pattern observed in the δ¹⁸O and δ²H values indicates that the deep groundwater infiltrated under colder climatic conditions than today, likely during glacial conditions. δ¹⁸O and δ²H values of deep groundwaters were detected as more negative than those of shallow (young) groundwaters. Accepting the correlation between stable isotope composition of Hungarian groundwaters and temperature, deep groundwaters have infiltrated in 6 to 8 ^oC colder climate than the current temperature. Their Ice-age infiltration is supported by the oldest ¹⁴C groundwater ages of 16 to 21 ka, close to the Last Glacial Maximum (LGM). Their ³H content being lower than the detection limit (<0.5 TU), also confirms the old groundwater ages. The oldest groundwaters and the lowest δ¹⁸O and δ²H values were found near the Koppány Stream, but they contribute only a small portion to the stream's baseflow.

Acknowledgements
The research presented herein was supported by the National Research, Development and Innovation Office (OTKA project grant number SNN 143868). The authors also acknowledge the financial support received by the Slovenian Research Innovation Agency (ARIS) for project N1-0309 and research program P1-0143.